I have used these several times and they work pretty good. I have worked the plenum area to remove the 4 hole style opening and make it more open. Helps flow and works better with spacers. I use a sawsall with metal cutting blades. Takes about 30 minutes and several blades to cut the lower plenum side and then do some smoothing with a die grinder.
Picture of plenum work but on a 429SCJ imntake.